31 October 1997

Gene test will identify the best bulls earlier

In the first of an occasional

series, we take a look at the

progress of biotechnology,

and its benefits for the cattle

industry. Emma Penny reports

REDUCED semen costs, better feed intake, less mastitis and disease concerns, improved fertility and reduced lameness.

All these benefits are promised by research at Edinburghs Roslin Institute.

The work is concentrating on finding the genes which influence these traits in dairy and beef cattle. The outcome will be that scientists will be able to test animals as soon as they are born to determine if they are genetically superior – a particular advantage for bull testing.

The technique should cut the number of bulls going into progeny tests, as superior animals will be identified much earlier. This means there is potential to test fewer bulls, be more convinced that they are the right animals, and so cut wastage from progeny testing, which costs £15,000-£20,000 a bull.

For producers, the benefits are that they will be able to use good bulls at an earlier stage, speeding up the rate of genetic progress. Lower testing costs should mean semen prices fall.

The crux, according to one of the institutes molecular biologists, John Williams, is that although all animals have genes for production, some are better than others.

"We need to look at high and low yielding cows and compare their genetic make-up. This should help us find the genes which influence production."

But it is not quite as easy as that. The bovine chromosome contains about 100,000 genes, only a few of which might be involved in a particular trait such as ensuring good calf growth.

John Woolliams, cattle breeding specialist at Roslin, says these production – or quantitative – traits are more complicated than others which, for example, determine whether an animal has horns. "Quantitative traits such as milk yield are controlled by a number of genes, rather than, say, just one which codes for horns or no horns."

Identifying those genes is tricky, but American researchers have already managed to identify some of the genes influencing quantitative traits – calling areas of the chromosome where these groups of genes occur quantitative trait loci. The five QTLs identified in the USA are overall milk yield, protein yield, fat yield, fat % and protein %, says Dr Williams.

"That means producers could select a bull which would allow them to fulfill milk contract specifications more accurately. For instance, a bull could be selected on the basis that it increases milk fat where butter production is the main aim."

Positive selection for desired characteristics will not penalise other vital traits, as animals will be selected for being good in all areas, rather than, for instance, selecting a bull just because he is better for feet and lameness, but perhaps less good for production. "Often, in conventional selection, one trait can be selected and improved, but progress fails in other areas."

"So far, only genes having a major effect on these production traits have been identified. There will be many more which have a smaller effect but it may not be worthwhile trying to identifying these." The work in the USA has identified several critical genes using markers, chemicals which help flag up a particular gene. But the American work is commercially confidential, which means it cannot be used to identify desirable sires in the UK.

But the law of averages would suggest that top bulls should all carry the desirable genes for production. And Mr Woolliams says it is a reasonable assumption that many of the USAs top dairy bulls will have been tested.

These two factors could help narrow the search field, he says.

"We are building up a semen bank as a source of genetic material, supported by the MDC, and will be mapping the genes of many of these bulls. We hope to be able to use this information to help identify the important genes – it would be surprising if these good genes were not prevalent in UK bulls."

A new herd set up at Roslin, funded by MAFF, MLC, MDC and the Biotechnology and Biological Sciences Research Council will also help identify vital QTL genes, says Mr Woolliams.

"We are setting up a Charolais cross Holstein herd. The breeds are as far apart as possible on characteristics such as milk and beef production. We should be able to identify precise genes involved in those traits and their location."

According to Dr Williams, while the Holstein is skin and bone, but produces high milk yields, the Charolais is fast growing and beefy, but with little milk. Besides the differences in feed conversion, liveweight gain and milk yield, mothering ability and calf vigour are also to be studied.

"There is such a big difference between the breeds that we are confident we can map many traits."

That includes traits influencing health. This will allow animals to be selected on a welfare-positive basis without biasing selection for other traits, says Dr Williams.

Mr Woolliams adds that in the past, the gains from genetic improvement were underestimated. "The benefits of genetic improvement are that it is permanent and cumulative. It is also cost-effective and welfare positive, with the aim of breeding a cow which is healthy and productive." &#42

Charolais x Holstein cattle at Roslin should help researchers identify genes controlling valuable production traits such as milk yield or growth rate.

John Williams… Comparing the genetic make-up of high and low yielding cows helps identify genes which influence production.

John Woolliams… Genetic improvement will help breed a cow which is healthy and productive.

WHAT THE BUZZWORDS MEAN

DNA Deoxyribonucleic acid – carries the genetic information in organisms.

Gene The biological unit of inheritance – a segment of DNA which provides the genetic information necessary to make a specific protein.

Genome All the genetic information which defines an organism. Bacteria and yeast have small genomes, while complex organisms like farm animals or humans have much larger genomes.

Chromosome The genetic information of a complex organism – its genome – is contained within packages called chromosomes. These packages of genes, basically long coiled chains of DNA, are found in the nucleus of cells. Each sperm and egg contains one set of chromosomes, but cells in the developing embryo contain pairs of chromosomes – 23 in a human – one inherited from the mother and one from the father.

Genome mapping Finding out where genes lie in an organisms genome. Identifying where these are means they can be located and may be used usefully in genetic improvement.

Markers These are like signposts, and flag up important genes. They are the same as DNA fingerprints and show up whether specific genes are present within that organism.

QTL Quantitative Trait Loci – regions of a chromosome which contain genes which promote high, or low, production. The Roslin trials hope to identify the QTLs associated with better performance.